U.S. Pat. No. 5,236,629 to Mahabadi et al., the disclosure of which is totally incorporated herein by reference, discloses a process for making submicron conductive polymeric particles useful as carrier powder coatings by a semisuspension polymerization process. In the semisuspension polymerization process, a mixture of monomers or comonomers, a polymerization initiator, an optional cross-linking agent and an optional chain transfer component are first bulk polymerized to the onset of gel-effect. The onset of gel-effect occurs when about 10 to about 50% of the monomers or comonomers are converted to polymer. Bulk polymerization is then stopped by cooling, and a conductive filler is added to the cooled, partially polymerized product. There is no disclosure or suggestion that the conductive filler can be added at any time other than following bulk polymerization of the monomers or comonomers.
Following addition of the conductive filler to the partially polymerized product, the partially polymerized product containing conductive filler is mixed to form an organic phase which is then suspended in water, with mixing, to form small particles, on the order of 10 microns or less. While still in suspension, polymerization is completed to obtain submicron sized particles, on the order of 0.05 to 1 micron in size, that are conductive.
As discussed in U.S. Pat. No. 5,236,629, the semisuspension process has advantages over other known prior art conductive particle formation processes such as grinding, precipitation and in situ particle polymerization. The advantages include the ability of semisuspension polymerization to yield particles of a small size that have a narrow particle size distribution and also the ability to better distribute the filler throughout the particle's polymer matrix. Semisuspension polymerization is also superior to straight suspension polymerization in allowing for the production of smaller particles that have a controlled size distribution.
However, a drawback to the semisuspension polymerization process when producing submicron particles is the tendency of the process to produce submicron sized particles that do not contain any conductive filler. This tendency is believed to be the result of a competing emulsion polymerization process that occurs in the aqueous phase of the suspension. Generally, both the monomers or comonomers and polymerization initiators are slightly soluble in water. Thus, minor amounts of these materials diffuse into the aqueous phase of the suspension and react to form polymer particles. Because the conductive filler remains in the organic phase of the suspension, these emulsion polymer particles contain no filler and are non-conductive.
When the use being made of the particles requires a high and uniform conductivity, such as is generally required for most conductive powder coatings and particularly for coatings in conductive magnetic brush systems, the presence of any amount of non-conductive particles creates a problem. These particles act as insulators in the powder coating and yield a coating of lesser conductivity than desired and which is also not uniform in conductivity throughout the coating. It would therefore be desirable to achieve a semisuspension process in which non-conductive particles are not produced.
In U.S. Pat. No. 4,330,460 to Hoffend et al., a process for preparing color toner particles by suspension polymerization is disclosed. The suspension polymerization process generally produces toner particles in the size range of about 2 to about 30 microns. The process also produces under-sized, unpigmented polymer particles that create problems in electrostatic transfer of the toner particles and in fusing of the toner particles to the image receiving substrate. Hoffend discloses eliminating the production of such unpigmented polymer particles by adding small amounts, less than 2,000 parts per million based on the weight of the aqueous phase (i.e., less than 0.2 wt.% ), of an inorganic salt to the aqueous phase of the suspension. The inorganic salt is disclosed to act as a scavenger which deactivates materials that would polymerize in the aqueous phase. In the suspension polymerization process of Hoffend et al., very small amounts of the inorganic salt had to be used in order to avoid unacceptably inhibiting the formation of polymer particles that contained pigment. The small amounts of inorganic salt added were disclosed by Hoffend et al. to be effective in preventing the formation of unpigmented polymer particles.
In the semisuspension polymerization process, the small amounts of inorganic salts suggested to be added by Hoffend et al. have proven to be ineffective in preventing the formation of non-conductive submicron polymer particles.